The central part of any laser device is the cavity wherein external energy is converted into coherent, monochromatic "laser" light. In this central cavity, optical resonances occur which determine, in large part, the efficiency of operation and quality of the light obtained from any laser. For example, in the industrially-important CO.sub.2 laser, an electrical discharge is often created in flowing gases. The excited gas thus created gives up its energy in the form of light in the laser resonator cavity. The design of this cavity has a significant impact on the structure of the resulting laser beam in terms of power distribution, mode structure, stability, etc. All these attributes are loosely referred to as "beam quality". For many industrial applications, high stability is needed in laser output. That is, the beam quality must be high and must remain so for long periods of time. Changing beam quality leads to changing focal characteristics of the beam and other problems. Hence, the amount of laser energy delivered to the work piece will vary and a predictable, reliable energy source has not been achieved.
For laser oscillation to occur, both ends of the laser resonator cavity must be partially or totally reflective at the wavelength of operation. The reflection characteristics of the ends of the laser resonator cavity must be critically designed, carefully engineered and skillfully constructed to be very stable. The relative alignment of reflecting devices at opposite ends of the laser resonator cavity must be precisely fixed and remain precisely fixed for as long as the laser is to be used. Should any misalignment of the cavity mirrors occur, from differential thermal expansion, mechanical vibration, etc. serious degradation can be expected in the performance of the laser.
One method of obtaining higher laser output power is to use a longer resonator cavity. Use of a longer cavity places a higher volume of excited lasing medium between the resonator mirrors, giving a longer path through which gain occurs. This leads to higher laser powers. However, longer resonator cavities cause the laser mirrors to be farther apart, making it more difficult to achieve and maintain precise mirror alignment.
Existing lasers have used a variety of methods to maintain mirror stability while increasing the output laser power. Large granite slabs have been used by Photon Sources, Inc. in their Turbolase T3000 laser and Alpha-Lase 250; various metal rods have been constructed to self-compensate for thermal changes, for example by Avco, Inc. in their Series 200 and HPL lasers, by Spectra Physics, Inc. in Models 971, 973, 975; and other mechanical stabilization means have been employed for example in the Series 9000 CO.sub.2 Laser manufactured by Control Laser, Inc. Obviously, such methods become more cumbersome and more expensive as longer resonator cavities are constructed and the spacing between the mirrors increases. As users demand higher laser powers, excellent beam quality and long term operating stability, the need to maintain precise mirror alignment will become crucial.